Polycarbonates are commercially important materials possessing excellent physical and chemical properties which are useful in a wide range of applications from non-opaque impact resistant sheets to shaped articles. The usefulness of the polycarbonates has been even further broadened in recent years, first by the development of a general application lamination and plastics coating technology and subsequently by the development of a lamination and plastics coating technology specific to the polycarbonates. The processes of these technologies attempt to combine the desirable properties of a polycarbonate with the properties of a laminate or plastics coating to produce a superior plastic article.
Many of the prior art lamination and plastics coating techniques have been directed towards improving the scratch- and mar- resistance of polycarbonate and other polymeric substrates, while preserving outdoor weathering characteristics and optical clarity. However, coatings may have application directed towards the improvement of a wide range of substrate characteristics, for example, solvent resistance or decorative appearance.
The acrylics, especially the polyfunctional acrylics, have been successfully used for some time in the plastics coating industry as a coating material for a wide range of substrates. Generally, the primary purpose of the acrylic coatings has been to improve weathering characteristics, scratch- and mar- resistance, and resistance to solvent attack, while preserving the non-opacity of the article.
U.S. Pat. No. 3,968,305 describes a synthetic shaped article having a mar-resistant polymer surface layer, with such a polymer surface layer consisting essentially of, in polymerized form, (a) 20 to 100 weight percent of a compound having a total of at least three acryloxy and/or methacryloxy groups linked with a straight chain aliphatic hydrocarbon residue having not more than 20 carbon atoms and, (b) 0 to 80 weight percent of at least one copolymerizable mono- or diethylenically unsaturated compound. This non-opaque surface layer is applied primarily to enhance scratch- and mar-resistance. It suffers from the fact that it generally has poor durability of adhesion after prolonged exposure to weathering.
U.S. Pat. No. 3,968,309 describes a molded article of plastic having on its surface a cured film of a coating material having a thickness of about 1 to about 50 microns comprising at least 30% by weight of at least one polyfunctional compound selected from the group consisting of polymethacryloxy or polyacryloxy compounds having a molecular weight of 250 to 800 and containing at least three functional groups in the molecule, and optionally, 0.01 to 5% by weight of a fluorine-containing surfactant and 0.01 to 5% by weight of a carbonyl photosensitizer. The surface characteristics which this coating material imparts to the article, are high surface hardness and minimized optical distortion.
Certain acrylic coatings have been found to be especially useful when applied to polycarbonate substrates. U.S. Pat. No. 4,198,465 describes a polycarbonate article having on its surface a UV cured coating comprised of the photoreaction products of (i) at least one polyfunctional acrylate monomer having from 2 to 4 functional groups attached to an aliphatic hydrocarbon residue containing 1-20 carbon atoms and optionally having ether linkages and/or substituted hydroxl groups; (ii) a photoinitiator; and (iii) resorcinol monobenzoate. The application of such a polyfunctional acrylic coating increases the scratch- and mar-resistance of the polycarbonate article without an increase in optical distortion.
It has now been found that a coating composition containing certain specific polyfunctional acrylic monomers in combination with certain polysiloxane-polyether block copolymers and azobisisobutyronitrile provides an excellent decorative plastic and durable UV curable coating for polycarbonate and other substrates. This decorative and durable coating is translucent, having a surface texture exhibiting a pattern of very regularly spaced parallel ridges which surface feels slippery to the touch.
The present invention provides certain polyfunctional acrylate ester monomer based UV-curable coatings for polycarbonates and other plastic substrates, a method for application of such coatings, and a resultant coated plastic article. These coatings adhere tenaciously and durably; are especially compatible with polycarbonate; are mar-, abrasion- and solvent-resistant; are not deleteriously affected by exposure to weathering; and provide a decorative, translucent appearance to a plastic article.